At 15:57 23 July 2019, Tango Eight wrote:
The smart guys are on O2 at 10,000 during the day and 5,000 at night.
Most humans are measurably impaired at 12,500.

T8

Much depends on your age, fitness, smoking habits, medication such as
beta-blockers, length of flight and the altitude at which you normally
live. At 10,000ft, you need 30% oxygen to function as well as normal if you
live at sea-level. Since air is 21% oxygen: that is a 50% increase. Even
10,000 ft is too high without supplemental oxygen for many middle-aged
pilots who should be using a pulsed-demand supply soon after take-off for
high flights. The law is set in much the same way as blood/alcohol levels.
If you stay within legal limits, it does not mean that you are flying as
safely as you could. For example: visual sensitivity at night is decreased
by 10 percent at 5,000 ft and by 30 percent at 10,000 ft. Even in the day
at 10,000 ft there are reductions in memory capacity, patience, focus,
endurance etc. Jean-Marie Clement's book has 39 pages on the subject!